Valving mechanisms may be used to control the flow of gas through a liquid. Such valving mechanisms are employed in systems which require a precisely timed release of gas in order to cause the gas to perform work or in order to provide a desired gaseous state within the environment in which the gas is released. Alternatively, the valving mechanism may be used to retain the gas until a time when the effects of the release will be minimal. A gas management valving mechanism may be a large scale device or may be formed using micromachining techniques, depending upon the desired application.
Air management is desirable in inkjet printing to prevent inkjet cartridges from "depriming" due to the accumulation of an air bubble in the ink flow path. Air bubble accumulation is a particular worry near a thermal inkjet printing head, which typically comprises a silicon chip containing an array of heating resistors which boil ink and expel it, through an array of orifices adjacent to the resistors and onto nearby paper. The ink to be expelled is typically at a small negative pressure with respect to atmosphere to prevent it from drooling out of the orifices, but too large a negative pressure can suck air in through the orifices, forming bubbles in the ink. In addition, heat from the boiling of the ink causes air dissolved in the ink to outgas and form small bubbles. These bubbles may coalesce in the ink over the silicon chip to form large bubbles which can impede ink flow, causing print quality to suffer. The impeding of ink flow by this air bubble is called depriming.
Trapped bubbles cannot simply float away from the inkjet chip because the inkjet pen typically requires a filter screen over the inkjet chip to prevent particles in the ink from clogging the inkjet orifices. The filter screen must be placed in the inkjet cartridge near the inkjet chip to reduce the likelihood that particles will be trapped in the volume between chip and screen during manufacturing. Typically, the screen is placed at the top of a "standpipe" region in which trapped air accumulates until the air bubbles become so large that print quality suffers.
Introducing a capability to remove the trapped air bubbles from the standpipe region can thus greatly increase the service life of the inkjet cartridge before print quality begins to suffer from mechanisms other than air accumulation.
A potential solution is described in U.S. Pat. No. 4,931,811 to Cowger et al., which is also assigned to the assignee of the present invention. The ink supply of an inkjet pen is connected to the thin film printhead by way of a large diameter standpipe. The diameter of an air accumulating section of the standpipe is sufficiently great to enable ink to pass through the standpipe, despite the presence of air in the air accumulating section. Large diameter air bubbles which form in the air accumulating section are deformed by suction force from the printhead, allowing ink to pass through the standpipe between the air bubbles and the walls of the standpipe. However, once the standpipe is completely filled with an air bubble which contacts the upper surface of the silicon chip, depriming can still be expected to occur.
Depriming continues to be a main contributor to premature failures of ink cartridges. Moreover, while the solutions described in Cowger et al. may provide an improvement within ink cartridges, the approaches may not be applicable to other systems in which gas-release management is desirable.
What is needed is a gas flow control device and method which achieve gas management without requiring movable components and which may be used in such applications as selectively releasing air through an ink supply of an ink cartridge.